In designing water-borne vessels to be propelled by wind, custom sails are fabricated for specific sailing craft and configurations. The custom sails are fabricated from a flexible and pliable sail cloth and are typically referred to as soft sails. While designers and sail makers provide efficient designs for specific sailing craft, soft sails deform ‘or luff’ in strong winds when for example the sails are angled acutely into the wind when sailing upwind.
In addressing shortcomings in efficiency dependent on sail trim with soft sails, rigid wing sails have more recently been adopted, particularly in racing sail boats. The rigid wing sail can be pivoted or slewed to adjust its angle of attack to the wind for the most efficient operation and propulsion of the water-borne vessel to which it is mounted. The rigid wing sail is generally of a symmetrical section which allows it to develop lift on either side according to whether the vessel is on port or starboard tack. However, a rigid wing sail suffers from at least the following drawbacks:    (i) it cannot decrease its area by ‘reefing’ in strong winds or increase its area by ‘unfurling’ in light winds;    (ii) a rigid wingsail cannot change its camber;    (iii) even when depowered or ‘feathered’ and placed parallel to the direction of the wind, a rigid wing sail is difficult to control and depower.